1. Field of the Invention
The present invention relates to positioning control systems and more particularly to an improved typewheel servo control system for precisely positioning a typewheel to a desired fine position after a selected coarse position is reached.
2. Description of the Prior Art
In conventional typewheel servo control systems, the characters of a typewheel are separated from each other by an angular distance which is represented by a fixed number of lines of an optical encoder disk. The encoder disk and the typewheel are coupled directly or indirectly to a servo motor shaft. Such character separations are interpreted by the servo control system as being fixed with respect to each other. However, in practice they may vary significantly due to factors such as optical disk tolerance; motor run-out, mechanical tolerances of the system and typewheel tolerance. As a result, typewheels of extremely tight tolerance must be used in order to achieve good print quality. In addition, optical disks with a large number of lines must be used to achieve acceptable resolution.
The background art known to the applicant at the time of the filing of this application is as follows:
U.S. Pat. No. 3,573,589, Position Servo System For A Motor Including Detenting at Destination, by J. Berry;
U.S. Pat. No. 3,644,814, Fine Positioning System, by G. Firkert;
U.S. Pat. No. 3,976,929, Device For The Exact Positioning of a Movable Part, by R. J. Rodier;
U.S. Pat. No. 4,345,192, Control System For Stopping Spindle At Predetermined Rotational Position, by Y. Kohzai et al; and
U.S. Pat. No. 4,429,267, Digital Positioning Systems Having High Accuracy, by J. R. Veale.